Normal cells cultured in vitro lose their proliferative potential after a finite number of doublings in a process described as cellular senescence (Hayflick and Moorehead, 1976). This phenomenon is not only well-established in human diploid fibroblasts based on the studies of Hayflick and Moorehead (1976), but it has also been validated by investigations using many additional cell types (Goldstein et al., 1990; Murano et al., 1991). These investigations document an inverse correlation between replicative senescence and donor age and a direct relationship between replicative senescence and donor species lifespan (Hayflick and Moorehead, 1976; Goldstein et al., 1990; Murano et al., 1991). In agreement with this association, cells from patients with premature aging syndromes, such as Werner's syndrome and Progeria, achieve a quiescent state much more rapidly than normal human fibroblasts. In this context, if similar senescence related changes occur in normal fibroblasts, albeit with delayed kinetics, these cell systems represent excellent models for studying senescence in vitro and identifying genes relevant to the aging process.
Senescence is characterized by changes in cell morphology, lack of responsiveness to mitogenic stimulation and irreversible growth arrest. However, cells can withdraw from the cell cycle and become non-dividing not only during senescence but also during the processes of DNA damage, apoptosis or terminal differentiation. While senescence is a time-dependent process (Campisi et al., 1995), terminal differentiation can be induced in a variety of cell types by appropriate treatment (Roberts et al., 1999). For example, terminal differentiation can be induced by cAMP treatment in melanocytes (Medrano et al., 1994). Gene expression analysis in terminally differentiated versus senescent melanocytes indicates both similarities and differences (Medrano et al., 1994). Although both pathways result in an elevation in p21 and an inability to phosphorylate ERK2, only the differentiated cells display elevated levels of p27 and the melanocyte-specific transcription factor (MITF) (Medrano et al., 1994; Smith and Pereira-Smith, 1996).
Human melanoma represents an excellent model for studying irreversible growth arrest and terminal differentiation, since these physiological changes can be chemically induced by IFN-β plus mezerein (MEZ) (Fisher et al., 1985; Jiang et al., 1994a). The induction of terminal differentiation in HO-1 human melanoma cells correlates with up-regulation of c-jun, jun-B, α5 Integrin, β1 Integrin, fibronectin, HLA Class I, ISG-54, ISG-15 and gro/MSGA as well as down-regulation of c-myc (Jiang et al., 1993a). To define the repertoire of genes differentially expressed during induction of irreversible growth arrest and terminal differentiation in human melanoma cells we have used a rapid and efficient differentiation induction subtraction hybridization (DISH) approach (Jiang and Fisher, 1993). Using this approach alone and in combination with high throughput screening strategies, microchip DNA arrays, a large number of novel genes of potential relevance to growth control and terminal differentiation have been identified and cloned (Jiang et al., 1995a, 1995b; Lin et al., 1996, 1998; Huang et al., 1999).
On the basis of the considerations described above, it is probable that specific differentially expressed genes may be present within a terminally differentiated cDNA library that also display modified expression during cellular senescence. To begin to identify these overlapping genes, a temporally spaced subtracted differentiation inducer treated HO-1 human melanoma library was screened with RNA isolated from senescent human fibroblasts. Such a screening protocol yielded twenty-eight known and ten novel cDNAs. Subsequent Northern and reverse Northern blotting analyses revealed differential expression of specific cDNAs. Expression of one of these cDNAs, Old-35 was restricted to terminal differentiation and senescence. In this context, this gene may contribute to pathways leading to growth arrest, a defining component of senescence and terminal differentiation.
Interferons (IFNs) comprise a family of related cytokines with diverse including antiviral, antiproliferative, antitumor and immunomodulatory activities (Stark et al., 1998; *Roberts et al., 1999). IFN studies have focused on two main areas; one involving the clinical use of IFN for therapeutic purposes (Gutterman, 1994), the other employing the IFN system as a paradigm to study the mammalian JAK/STAT signaling cascade (Darnell et al., 1994) that leads to IFN-stimulated gene (ISG) activation. To date, the most extensively studied ISGs include RNA-activated protein kinase (PKR), the 2′-5′ oligoadenylate synthetase and the MX proteins (Stark et al., 1998, *Der et al., 1998).
Post-transcriptional regulation of mRNA levels is a pivotal control point in gene expression. Early response genes, such as cytokines, lymphokines and proto-oncogenes are regulated by a cis-acting adenylate-uridylate-rich element (ARE) found in the 3′ untranslated region (UTR) of the mRNA (Caput et al., 1986; Shaw and Kamen, 1988; Chen and Shyu, 1995; Myer et al., 1997). Currently, three classes of destabilizing elements have been identified: AUUUA-lacking elements and AUUUA-containing elements grouped into those with scattered AUUUA motifs (such as proto-oncogenes) and those with overlapping AUUUA motifs (such as growth factors) (Chen et al., 1995; Myer et al., 1997). A transfer of 3′UTR containing ARE to 3′UTR of a stable message, such as β-globin, targets this very stable mRNA for rapid degradation (Shaw and Kamen, 1988). In contrast, the removal of an ARE stabilizes an otherwise labile message (*Miller et al., 1984; *Lee et al., 1988).
The present studies describe the cloning and initial characterization of a novel gene, Old-35, from a terminally differentiated human melanoma cDNA library. mRNA stability studies document that Old-35 mRNA, which contains ARE elements, may be stabilized in H0-1 cells by treatment with IFN-β and IFN-b+MEZ. Based on the growth suppressive effect of IFN-β on HO-1 cells, as well as the increased stability of Old-35 during confluence and senescence, it is possible that this gene plays a prominent role in growth suppression induced by this cytokine. Further experimentation is required to define the precise role of Old-35 in IFN signaling, terminal differentiation and cellular senescence. Full-length cloning and subsequent protein analyses should provide insights into the function of this potentially important gene in the processes of aging and differentiation.
Since the processes of terminal differentiation and senescence exhibit strikingly similar characteristics, it is possible that related and overlapping genes and gene expression changes associate with and mediate both of these phenomena. Old-35 was isolated by screening a subtracted human melanoma cDNA library enriched for genes related to growth arrest and terminal differentiation with RNA from senescent human fibroblasts. This cDNA encodes an IFN-β inducible gene expressed during different types of growth arrest including confluence, senescence and terminal differentiation. Old-35 RNA exhibits increased stability in IFN-β and INF-β+MEZ treated H0-1 human melanoma cells. Steady-state mRNA for Old-35 is also highly expressed in heart and brain, human tissues without active regenerative properties. Judging from the pattern of Old-35 expression, it is possible that this gene may play a prominent role during growth arrest and in this context contributes to the important processes of senescence and terminal differentiation.